The invention relates to a polymeric gel electrolyte.
The invention further relates to a method of preparing such an polymeric gel electrolyte.
The invention still further relates to a rechargeable lithium battery comprising such a polymeric gel electrolyte.
Due to its small size and weight, high energy density and high voltage, a battery having a lithium-based anode, or lithium battery for short, is an attractive source of electric energy for portable and/or hand-held electr(on)ic equipment in particular. Such a battery comprises a lithium-based anode, a cathode, and disposed therebetween, an electrolyte.
In the case of primary lithium batteries, a highly ion-conductive liquid electrolyte can be suitably used. However, when employed in a rechargeable (also referred to as secondary) lithium battery such a liquid electrolyte gives, upon charging, rise to dendritic growth which in turn leads to short-circuits. Dendritic growth is less when a solid electrolyte is used. However, the ion-conductivity of a solid electrolyte is insufficient for many practical purposes such as when a battery is to be rapidly (dis)charged. In an attempt to combine liquid and solid electrolytes in an advantageous manner, the use of polymeric gel electrolytes, i.e. a polymer which is gelled by means of an ion-conductive liquid, has been proposed.
In this respect, reference is made to U.S. Pat. No. 5,501,921 which discloses polymeric gel electrolytes comprising a cross-linked polymer and an ion-conductive liquid. The cross-linked polymer is obtained by cross-linking functionalized alkane monomers each having 2 to 100 carbon atoms and each having at least one polymerizable functional group. The total number of polymerizable functional groups is selected such that the polymer electrolyte is substantially chemically inert when brought into contact with a lithium anode.
In practice it may happen that when such a known polymeric gel electrolyte is employed in a rechargeable battery and the battery is repeatedly charged and subsequently discharged, for example at a 0.2 C rate (a y C rate means that the battery is (dis)charged to its full capacity in yxe2x88x921h), said battery short-circuits after a small number of repetitions. Inspection of the short-circuited battery shows that dendrites have grown through the polymeric gel electrolyte.
It is an object of the invention to provide novel polymeric gel electrolytes which do not have the abovementioned disadvantages, or at least to a lesser extent. When employed in a rechargeable lithium battery, the polymeric gel electrolyte should have a high ion-conductivity, should not react with the lithium anode and should effectively suppress the dendritic growth which leads to short-circuits when the battery is repeatedly charged and discharged.
This object is achieved by a multi-phase polymeric gel electrolyte having an ion-conductivity of more than 1 mS/cm and comprising:
(a) a continuous solid phase predominantly comprising a polymer obtainable by polymerizing functionalized C2-C100 alkane monomers or alkylene oxide monomers having the formula {(CH2)pxe2x80x94O}m(II) wherein m=2, 3 or 4 and p=3, 4, 5 or 6 formule (II) each functionalized with at least one polymerizable group, and
(b) a continuous liquid phase interpenetrating said solid phase and predominantly comprising an ion-conductive liquid containing a solvent and a lithium salt at least partly dissolved therein,
wherein said solid phase has a microscopic network structure which is selected such that when a rechargeable battery is formed by disposing said multi-phase polymeric gel electrolyte between a lithium metal anode and a lithium cobaltate cathode said rechargeable battery has a capacity per unit surface area of at least 3 mAh/cm2 and is capable of being fully charged and discharged at least 20 times at a 0.2 C rate without short-circuiting.
The extent to which a polymeric gel electrolyte suppresses dendritic growth is measured by repeatedly fully charging and discharging a rechargeable lithium battery having a capacity per unit surface area of at least 3 mAh/cm2 and comprising said polymeric gel electrolyte disposed between a lithium metal anode and a lithium cobaltate cathode. If the said battery can be fully charged and discharged at least 20 times at at least a 0.2 C rate without short-circuiting, the solid phase of the polymeric gel electrolyte has a suitable microscopic network structure and the suppression of dendritic growth is considered to be effective.
Surprisingly, it is found that the ability of a polymeric gel electrolyte to suppress dendritic growth when employed in a rechargeable battery is significantly improved if a multi-phase polymeric gel electrolyte is employed in which the polymer, being part of a solid phase, and the ion-conductive liquid, being part of a continuous liquid phase, interpenetrate on a microscopic scale. After all, one would expect dendritic growth to be minimal if a one-phase polymeric gel electrolyte in which the polymer and the liquid interpenetrate on a molecular scale is employed. However, contrary to this expectation, suppression of dendritic growth is found to be most effective if the (polymer of the) solid phase is selected such that it has a microscopic network structure of a particular morphology.
As a typical example, polymerizing a composition composed of 40 wt. % decanedioldiacrylate and 60 wt. % 1 M LiPF6 in a 1:1 (v/v) mixture of ethylenecarbonate and diethylcarbonate produces a multi-phase polymeric gel electrolyte having an ion-conductivity of 1.2 mS/cm which, when employed in a lithium metal battery, allows said battery to be fully charged and subsequently discharged more than 40 times at a 0.2 C rate without short-circuiting.
The invention is based on the recognition that the occurrence of dendritic growth leading to short-circuits is related to a degradation of the polymeric gel electrolyte. The degradation is due to a reaction between (neutral) lithium and the polymer occurring at the interface between polymer and liquid. By providing a multi-phase polymeric gel electrolyte in which the polymer is phase-separated from the ion-conductive liquid on a microscopic scale the surface area of the interface and, consequently, the rate at which the degradation reaction occurs is reduced.
The continuous solid phase of the multi-phase polymeric gel electrolyte in accordance with the invention has a microscopic network structure. It extends throughout the entire polymeric gel electrolyte in all directions thus giving the polymeric gel electrolyte a mechanical support similar to that obtained for a one-phase polymeric gel electrolyte.
It is emphasized that, in the context of the invention, the term network structure refers to a network of microscopic dimensions and not to the network of covalently bonded atoms of a cross-linked polymer.
As already mentioned above, the morphology of the microscopic network structure of the solid phase is an essential parameter with respect to the suppression of dendritic growth leading to short circuits because it determines the morphology of the liquid phase which is the complement to that of the solid phase.
Although it is not to be interpreted as limiting the scope of the invention, electron micrographs taken of the surfaces of films of multi-phase polymeric gel electrolytes show that a suitable network structure is one which is composed of more or less spherical particles having a characteristic diameter of 0.1 to 0.5 xcexcm. On the other hand, network structures of spherical particles having a characteristic diameter of 0.5 xcexcm to 1.0 xcexcm or larger have by and large been found unsuitable. Obviously, apart from the size of the particles there should be a sufficient number of them. That is the network structure is to be sufficiently dense. The density of the microscopic network may be simply increased by increasing the weight ratio of solid to liquid phase.
As is well known to those skilled in the art, there are many parameters which influence the morphology of a multi-phase material. Examples of such parameters in the context of the present invention include, but are not limited to, the polymer, the ion-conductive liquid and the relative amounts in which these components are used. Another parameter, one which is independent of the composition of the polymeric gel electrolyte, is the method of preparing the polymeric gel electrolyte.
The liquid phase interpenetrates the solid phase. Because it is continuous, the lithium ions present therein are able to move throughout the polymeric gel electrolyte.
The considerations regarding the weight ratio of polymer (solid phase) to ion-conductive liquid (liquid phase) in a multi-phase polymeric gel electrolyte are basically the same as those in the case of a one-phase polymeric gel electrolyte. The desired ion-conductivity (higher at larger weight ratios) is to be balanced against the desired mechanical strength (higher at smaller weight ratios).
In accordance with U.S. Pat. No. 5,501,921, C2-C100 alkane monomers are employed so as to avoid a chemical reaction between the lithium anode and the polymer. The functionalized alkane monomers may form a polydisperse or monodisperse composition. They may each be branched or unbranched, and, if desired, functionalized with a small number of other groups. For example, a xe2x80x94CH2xe2x80x94 unit may occasionally (not more than once per 5 carbon atoms of the alkane monomers in order to minimize reactivity with respect to lithium) be replaced by an xe2x80x94Oxe2x80x94 unit or an amylene unit such as phenylene, naphtphylene, biphenylene, and 4,4xe2x80x2-isopropylidenediphenyl.
The alkane monomers are each at least functionalized with one polymerisable group. Suitable polymerisable groups include isocyanate, epoxy groups or ethylenically unsaturated groups such as vinyloxy, acrylyl, methacrylyl or styryl groups. Preferred are methacrylyl or acrylyl groups. The thiolene system is also suitable.
The liquid phase predominantly comprises an ion-conductive liquid. Since the electrolyte is to be used in a lithium battery, a lithium salt is used which is dissolved in a solvent.
Lithium salts known per se are suitable and include LiBr, LiSCN, LiI, LiCl, LiClO4, LiBF4, LiAsF6, LiCF3COO, LiCF3SO3, and LiPF6.
Solvents which are non-aqueous, aprotic and polar can be suitably used and include solvents known per se such as 1,3-dioxalane, 2-methyltetrahydrofiran, xcex3-butyrolactone, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, and, in particular carbonates such as diethylcarbonate, dimethylcarbonate, ethylenecarbonate and propylenecarbonate or mixtures thereof.
The lithium salt concentration is preferably as large as possible since this will lead to a higher ion-conductivity. However, if the solubility limit is exceeded the ion-conductivity does not increase anymore. A suitable weight ratio of lithium salt to solvent is 0.6 to 1.0.
The multi-phase polymeric gel electrolyte can be moulded into any shape such as a layer, provided on a substrate or prepared as a self-supporting film. When employed in a rechargeable lithium battery, the layer thickness is suitably selected between 5-200 xcexcm, but is preferably between 25-100 xcexcm, or better still between 30-60 xcexcm.
U.S. Pat. No. 4,654,279 discloses a solid polymer electrolyte comprising an interpenetrating network of two co-continuous phases, one of these phases being a cross-linked polymer providing a mechanical support matrix, the other being an ion-conductive phase comprising a metal salt and a salt complexing liquid polymer. However, the polymers disclosed therein are not obtained from functionalized C2-C100 alkane monomers as a result of which, in accordance with U.S. Pat. No. 5,501,921, the polymer electrolytes disclosed in U.S. Pat. No. 4,654,279 are not chemically inert with respect to lithium. The ion-conductivity of the electrolytes disclosed therein is significantly lower than that of the electrolytes in accordance with the present invention. Also, U.S. Pat. No. 4,654,279 is completely silent on the problem of dendritic growth, let alone on solutions which may alleviate said problem. Furthermore, a rechargeable lithium battery is not explicitly disclosed.
In a particular embodiment of the multi-phase polymeric gel electrolyte in accordance with the invention, the polymer is a cross-linked polymer. In the context of the invention, cross-linked is understood to mean chemically cross-linked by means of chemical bonds as opposed to physically cross-linked due to entanglement of polymer chains. A cross-linked polymer provides excellent mechanical strength across a wide range of temperatures and for extended periods of time. A film made thereof is sufficiently flexible and compressible to take up changes in shape of the anode or cathode of a battery which may occur as a result of (dis)charging. A cross-linked polymer is obtainable by polymerizing monomers of which at least a certain number is provided with more than one polymerizable group. Suitable cross-linked polymers include cross-linked epoxides, polyurethanes, poly(meth)acrylates, polystyrenes, and polyacrylonitriles.
Preferably, cross-linked poly(meth)acrylates are used which may be obtained by radiation-polymerizing corresponding (meth)acrylate monomers. Electron and gamma radiation are suitable and have the advantage that they do not require an initiator compound. However, photo-polymerisation using UV radiation in combination with an UV-sensitive initiator compound, examples of which are well known to those skilled in the art, are preferred. In order to obtain an electrolyte comprising a solid phase having a suitable microscopic network structure, the use of cross-linked polyacrylates and in particular polydiacrylates is preferred.
In accordance with a preferred embodiment of the invention, the multi-phase polymeric gel electrolyte comprises 30 to 65 wt. % of a polymer obtainable by polymerizing alkane monomers represented by the formula (I) 
wherein n=8, 10 or 12 and R=H. When combined with conventional ion-conductive liquids, said polymers are capable of forming a multi-phase polymeric gel electrolyte comprising a solid phase having a microscopic network structure of a suitable morphology. If said alkane monomers are employed in the method in accordance with the invention described hereinbelow, a microscopic network structure in accordance with the invention is obtained.
The inventor has observed that at least if the polymeric gel electrolyte of this preferred embodiment is used, the limiting factor with respect to service life of the battery in which said electrolyte is employed is the cycling efficiency. Since the cycling efficiency is not 100 %, a part of the lithium anode is sacrificed during each discharge and the battery ceases to be operative if all the lithium of the anode is sacrificed. Since this happens before the battery short-circuits, the operation of the battery is inherently safe.
Preferably, the ion-conductive liquid is a lithium salt dissolved in an alkane carbonate such as a mixture of diethylcarbonate and ethylenecarbonate.
The invention further relates to a method of preparing a multi-phase polymeric gel electrolyte in accordance with the invention. The method comprises the steps of:
(1) providing a one-phase polymerizable composition comprising:
(a) functionalized C2-C100 alkane monomers or alkylene oxide monomers having the formula {(CH2)pxe2x80x94O}m(II), wherein m=2, 3 or 4 and p=3, 4, 5 or 6 each functionalized with at least one polymerizable group, and
(b) an ion-conductive liquid containing a solvent and a lithium salt at least partly dissolved therein, the alkane monomers being selected such that said one-phase polymerizable composition undergoes a phase separation during polymerization thereof,
(2) polymerizing said one-phase polymerizable composition, thereby forming the multi-phase polymeric gel electrolyte in accordance with the invention.
The method renders the multi-phase polymeric gel electrolyte readily processible even when, and in particular if, the polymer used is a cross-linked polymer.
The one-phase polymerizable composition may be formulated as a liquid or paste which may be processed by means of conventional printing and coating techniques thus allowing, for example, self-supporting films and thin coatings to be routinely made.
During polymerisation the composition undergoes phase separations. Phase separation is brought about by selecting alkane monomers which are miscible with the liquid in the monomeric state but not in the (pre)-polymeric state. In the art this is known as reaction induced phase decomposition. In order to arrive at a suitable microscopic network structure, a phase decomposition of the spinodal type is preferred. Examples of suitable alkane monomers and polymerizable groups have already been described hereinabove. The conditions under which the polymerization is to be carried out depend on the particular polymerizable group used and are well known to those skilled in the art.
The invention still further relates to a rechargeable lithium battery comprising a multi-phase polymeric gel electrolyte in accordance with the invention. Embodiments of suitable polymeric gel electrolytes and methods of manufacturing such suitable electrolytes are described hereinabove.
Cathodes of a conventional type which include oxidic compounds capable of intercalating and deintercalating lithium may be suitably used. Preferred cathodic materials are LiCoO2, i.e. lithium cobaltate, LiNiO2, LiMn2O4 and LixMnO2(O less than xc3x97 less than 0.5).
Suitable anodic materials are of a conventional type and include lithium alloys such as Li:Al, Li:Hg, Li:Pb, Li:Sn and Wood""s alloys, and composite materials of lithium and a carbon compound such as polyacetylene or graphite. Because of its high energy density lithium metal is preferred.
The battery (cell) may be of the cylindrical type, the sheet type, the button type or any other suitable conventional type.